Actually you can repeat that leve (with the same rewards) next reset. So long as you don't trade in the leve.

Example:
- You D/C and it's insta-fail
- Tomorrow guildleves reset
- You go up to the counter, chat with NPC so he resets your leves (DO NOT TRADE IN!!!)
- Open your journal, all "failed" leves have now reset with the same rewards as before
- Reattempt
- Enjoy